The old-timer
The man remembers the advice of an old-timer from Sulphur Creek who warned him against traveling alone in the Yukon when the temperature is lower than fifty degrees below. The man first scoffs at this advice when he adeptly handles his first accident, but later understands the wisdom in the old-timer's caution: man is not instinctively fit for the harsh, indifferent environment of the Yukon.
The boys
The man is trying to meet "the boys" by six o'clock at night. Presumably, they are prospecting for gold. Though they never appear in the story, the boys (and the man) are examples of the lower-class characters naturalism turned its attention to; only men without much to lose would risk their lives in the harsh Yukon.